Several devices have been used in the past utilizing a so-called "vanishing technique" to promote and facilitate the learning of written material. The principle of this technique is to cause the written material to gradually disappear. For example, the technique may be performed by writing the material to be learned on a blackboard with chalk and then erasing letters, then words, then phrases and finally the material in its entirety.
The vanishing technique may also be used by writing the material to be learned with a stylus upon a "magic slate"which is a common device having a backing sheet coated with a wax-like substance overlaid with a relatively translucent impression sheet and a transparent wear-resistant covering sheet. When the stylus is applied to the covering sheet, the pressure at the point of the stylus is transmitted through to the impression sheet and causes the impression sheet to adhere or bond to the backing and the writing to appear on the translucent impression sheet through the covering sheet. By gradually lifting the impression and covering sheets, the impression sheet is separated from the backing sheet to achieve progressive disappearance of the written material. Manifestly, the material written by the stylus disappears along the moving line of separation of the impression sheet from the backing sheet. As in the case of the use of a blackboard, above described, the written material cannot be reclaimed without writing it again on the magic slate.
In one specialized form of magic slate device adapted to cause selective disappearance of written material, shown in U.S. Pat. No. 3,135,059, a whole packet or set of backing sheets having holes of various sizes arranged in a pre-selected manner is covered by an impression sheet. When a stylus is used to create written material along guide lines permanently affixed to the impression sheet, the entire amount of written material to be learned appears on the impression sheet. Thereafter, as each backing sheet is separated in sequential order from the rear of the set, progressively greater amounts of the written material, i.e., The amounts impressed on each backing sheet through the holes in the sheets above, are deleted from the impression sheet.
A variation of the teaching instrument described above is shown and described in U.S. Pat. No. 3,613,266. Therein a sheet of printed or written material is attached to the slate for preliminary viewing by a student. After such preliminary viewing, the written material is folded out of the student's view, and he is required to test his recollection by writing his impressions with a stylus on the top sheet of the slate device. However, the stylus-written material is obscured to the student due to the fact that a fully opaque, non-bonding sheet is placed over the entire impression sheet of the slate. The student's writings are thus maintained invisible to him until the fully opaque sheet is lifted.
The principle upon which the foregoing devices operate is the use of a "magic slate" which causes stylus-written material to appear upon an overlay pressed against a backing sheet. When the overlay is lifted, the bond between the overlay and the backing sheet is eliminated and the written material destroyed. Moreover, any selected portions of the written material which are thus eliminated are subtracted from a student's view in a manner which prohibits further reference to the whole of the original writing.
Accordingly, it is an object of the present invention to provide an improved teaching apparatus and method which are adapted to permanently retain the written material to be studied while providing progressive obscuration.
It is another object of this invention to provide an improved teaching apparatus which is adapted to be transferred from one piece of written material to another or used with various materials.
It is another object of this invention to provide an improved teaching apparatus and method which include the use of a transparent overlay which does not require a permanent bond to a specific piece of written material to be studied.
It is another object of this invention to provide an improved teaching apparatus and method which may be used to progressively obscure the written material to be learned in a variety of sequences.
It is another object of this invention to provide an improved teaching apparatus and method which progressively obsure portions of the written material to be learned in mathematically random sequences.
It is another object of this invention to provide an improved teaching apparatus and method which are adapted to improve the confidence and accuracy of a student of the written material to be studied.
Other objects and advantages of the present invention will become apparent from an examination of the following detailed description of the invention, the drawings and the appended claims.